1. Field of the Invention
The present invention relates to a power steering system, which provides steering assisting force to a steering shaft.
2. Description of Related Art
Generally, in an electric power steering system, a torque sensor converts a torsional torque of a steering shaft, which is generated by steering of a steering wheel connected to the steering shaft, to an analog signal (voltage) and outputs it to an electronic control unit (ECU). In the ECU, an analogue to digital (AD) converter circuit converts the analogue signal to a corresponding digital value.
Therefore, when there is a difference between a reference voltage of the torque sensor and a reference voltage of the AD converter circuit, the measured torque value becomes erroneous and is not accurately transmitted. Thus, the voltage of the torque sensor and the voltage of the AD converter need to be matched with each other.
Japanese Unexamined Patent Publication No. 2001-88728 discloses such an electric power steering system. In the disclosed electric power steering system, a reference voltage of a torque sensor and a reference voltage of an AD converter circuit are supplied from a common power source to address the above disadvantage.
However, since the torque sensor and the ECU are distant from each other, for example, a voltage drop in electric lines, which connect between the torque sensor and the ECU, can occur, so that it is difficult to match the reference voltage of the torque sensor and the reference voltage of the AD converter with each other.
Furthermore, precise relative adjustment between the torque sensor and the ECU is required, and relatively high tolerance is required for the performance of the torque sensor. As a result, manufacturing and assembling costs are increased.
Furthermore, there are many difficulties in implementing of two or three torque sensors in the power steering system.